In this image, the longer growing season near cities and close to the water is indicated in red. Credit: Dr. Andrew Elmore/University of Maryland Center for Environmental Science

Mid-Atlantic suburbs can expect an early spring thanks to the heat of the big city

If you’ve been thinking our world is more green than frozen these days, you’re right. A recent study has found that spring is indeed arriving earlier – and autumn later – in the suburbs of Baltimore and Washington, D.C. The reason? The urban landscape traps heat in the summer and holds it throughout the winter, triggering leaves to turn green earlier in the spring and to stay green later into autumn. The result is a new, extended growing season.

Scientists used high-resolution satellite data collected over the past 25 years to look at the number days that trees have green leaves in the forests of the Mid-Atlantic. The study found that urban heat islands affected the growing season in areas within 20 miles of the city. As a result, gardeners may have more time to grow their vegetables and plant new varieties.

The longer growing season also has a profound impact on forests. Forests are, in effect, the world’s air filters. Green leaves on trees turn carbon dioxide – a greenhouse gas that traps heat in our atmosphere – into oxygen. Carbon dioxide also helps trees grow since they use energy from the sun to convert the gas into plant matter. A longer growing season could change how quickly forests grow and increase the amount of carbon dioxide taken out of the atmosphere.

“Everything changes when the leaves turn green,” said the study’s lead author Dr. Andrew Elmore of the University of Maryland Center for Environmental Science. “Trees start pumping water into the atmosphere. They take carbon dioxide out of the atmosphere. They make sugars and build plant tissue. It’s as if the entire landscape goes from exhaling to inhaling.”

The study also pinpointed other factors that influence the timing of spring and autumn in areas outside the influence of urban heat islands, including the elevation of the landscape, proximity to tidal water, and cold air drainage in small valleys.

Not all forests are the same, however, and predicting which forests will grow faster during a longer growing season requires detailed satellite measurements. This study is the first to apply high-resolution satellite data to the problem. “We are trying to understand how forests function so we can understand how they might respond to global warming,” said Dr. Elmore. “With more detailed data, we can do better job of predicting what might happen to a forest impacted by urbanization, for instance.”

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The study, “Landscape controls on the timing of spring, autumn, and growing season length in mid-Atlantic forests,” was published in the February issue of Global Change Biology by Andrew Elmore and Steven Guinn of the University of Maryland Center for Environmental Science, Burke Minsley of the U.S. Geological Survey, and Andrew Richardson of Harvard University.

UNIVERSITY OF MARYLAND CENTER FOR ENVIRONMENTAL SCIENCE

The University of Maryland Center for Environmental Science unleashes the power of science to transform the way society understands and manages the environment. By conducting cutting-edge research into today’s most pressing environmental problems, we are developing new ideas to help guide our state, nation, and world toward a more environmentally sustainable future through five research centers—the Appalachian Laboratory in Frostburg, the Chesapeake Biological Laboratory in Solomons, the Horn Point Laboratory in Cambridge, the Institute of Marine and Environmental Technology in Baltimore, and the Maryland Sea Grant College in College Park. www.umces.edu

UPDATE: kadaka writes in comments-

Paper found, can be freely downloaded, 3.25MB pdf.

This is an Accepted Article that has been peer-reviewed and approved for publication in the Global Change Biology, but has yet to undergo copy-editing and proof correction. Please cite this article as an “Accepted Article”; doi: 10.1111/j.1365-2486.2011.02521.x

58 thoughts on “City UHI makes spring bloom earlier”

Although the article does not directly say it, it is also likely that increased CO2 concentration in urban areas has played a role in extending the growing season. It cannot be said because that gets too close to admitting that additional CO2 has some beneficial effects.

The study found that urban heat islands affected the growing season in areas within 20 miles of the city.
Perhaps we need to redefine ‘rural’ WRT UHI. It is easy to conceive of a rural site, unchanged, with the exception of a city only 10 miles away. That the encroachment of effect from UHI has a 20 mile radius, should exclude many sites thought to be unaffected.

I wonder if trees put more water out than they take in? They split water to make O2 as part of photosynthesis. And they require water for growth. Here in New Orleans it is particularly obvious because we have a lot of warm weather but the plants all have a huge growth spurt after a heavy rain.
Water is released as starch and cellulose are synthesized by the plant though, so I don’t know what the net balance is.

This is ridiculous: the Team and the IPCC have clearly stated UHI is grossly overstated and should be ignored as a factor in interpreting historical temperature data.
Look at what Wikipedia, the font of all knowledge on global warming, has to say about UHI:
“Despite concerns raised about its possible contribution to global warming, comparisons between urban and rural areas show that the urban heat island effects have little influence on global mean temperature trends”.
The fact that most temperature monitoring stations are located in urban areas is therefore irrelevant.

This is an Accepted Article that has been peer-reviewed and approved for publication in the Global Change Biology, but has yet to undergo copy-editing and proof correction. Please cite this article as an “Accepted Article”; doi: 10.1111/j.1365-2486.2011.02521.x

ABC News ran a report Wednesday on their nightly news broadcast saying it was green house gases causing our early spring, and spring would be earlier and earlier as greenhouse gas emissions rise and the world continues to warm.
The same broadcast they also had a story on this year’s mild and dry winter, caused by the same culprit, AGW.

Another hey its reality study…
Yes warmer temps help plants grow which in turn means more, bigger plants… which in turn means more CO2 being used and converted to O2… also means more food produced. Why can’t I get 100k in grants to state well known facts?

“We are trying to understand how forests function so we can understand how they might respond to global warming,” said Dr. Elmore.
Yup, he just had to say that. But urbanization is not “global warming.” Oh, well. At least this was based on actual observation, no models to be found.

Notice that the map is in growing days not in delta T. However a change from a growing season of 150 days to a growing season of 210 days (2 months) is a heck of a lot for a distance of about 10 miles.

Apologies for the off-topic question, or if this serious issue has been dealt with somewhere else (impossible to read it anyway!), but what’s up with the the pale grey, all-caps and sans-serif microscript in the comments section? It’s as if someone combined all typographical boo-boos which make texts unreadable. Screen reading speeds are already lower by as much as 30% compared to traditional black type on white paper and the added tiny point size, harder to read sans-serifs and the low contrast between type and screen background make me suspect a psy-ops from the Warmies. Would have DeSmog’s mark of “sophistication,” methinks.
On the other hand, I think this alarming typographical blight appeared first on Willis’ article. That’s enough evidence to collectively blame Willis, I rather than to flog the Warmist dead horse. Blaming Willis is a hallowed custom here and results in colourful zingers from him which make for much better reading then the anemic whining and sniffling from the Warmies.

Great article! However, the cities themselves appear to be coded in gray with no growing season indicated. But the suburbs do tend to be oranger than the rest of the map. Contrary to the caption, there is no actual red on the map that I can find.
Another problem is that the cities tend to be at lower elevations than most of the countryside, and therefore run warmer already. A better map would adjust for altitude. As you go west, the elevation rises rapidly, but if the Delmarva peninsula is relatively flat it may be the best comparison. What’s its max elevation? For some reason, there is also a lot of gray in the western hills.
OT, but one thing that has always bothered me is whether Delmarva has an older geographical name that isn’t based on the colonies that happened to split it.

You only have to listen to any Weather Forecast. Every one states that the temperatures will be higher in the Cities and large towns than in the countryside. Usually at least a couple of degrees difference..
Coutrysides always have the Frost first. It is so obvious and yet so ignored.

UHI, solar activity, CERN, natural variation – so much thrown out before looking around for what could be left. All to explain a short-term temperature rise in populated land areas where cooling exists elsewhere and the atmosphere and oceans show next to nothing in trend. There’s a Shakespeare comedy in here somewhere (Much Ado About Nothing X Midsummer Night’s Dream) – unless tragedy ensues.
It’s getting so obvious all the action is about to move to acidification. Their minds are made up. They just need the right excuses.

“We are trying to understand how forests function so we can understand how they might respond to global warming,” said Dr. Elmore.
And yet they still can’t make the leap that this same UHI is why urban weather stations showed warming. Rural stations showed little to no warming. They are halfway there. Keep going with the thought process guys.

Since the study area is near the ocean and tends to be more humid than other areas, say Arizona, Colorado or parts of California, I wonder if the affect is more pronounced there. I think that it’s possible that some urban areas, Denver for example, where large masses of cold, dry air blow in and the temps stay below freezing for longer periods, may not see the same effect. YMMV.

It is easy to prove to yourself that UHI is real. Simply go to Google and type in “your local city, Wunderground temperature”, then scroll down to the bottom and you will find a number of temperature measurement stations near you. You can study the individual measurement stations by clicking on the name of each station; among other things a location map will appear. You can try this for several weather conditions and times of day but you will find the temperature range to be about 5 degrees F. By studying the data you can eliminate any station(s) that is/are obviously incorrect.
There are also flyover data, including Paris France, showing how UHI rises and then drops as one flies over buildings then parks and open areas. The UHI impact lasts for 25 miles or so.
The problem that I see is many of the temperature measurement stations used by the Climatologists are located in and near cities, airports and other developed locations. The UHI effect decreases as one leaves the developed areas, reaching normal climate temperature in 25 miles or so. The temperature data in the UHI areas is then homogenized by Hansen et al. This number fiddling spreads the UHI effect over an area much greater than 25 miles. Hansen makes so makes so many numeric mistakes, I no longer trust him to do anything correct. Stephen McIntyre has wiped his butt many times yet Hansen never seems to learn to review his work.

I’m calling BS on this. Since I live here, I feel I can comment. The whole area is filled with asphalt roads and red brick buildings. They definitely trap heat on warm summer nights. However, when winter rolls in with any sort of wind, we basically do what the prevailing wind does. Our temp just swung 30F in a couple days because of a warm front. And went right back down with the cold front last night. I’m downtown, so, I get the maximum heat island effect. I’m saying that the heat island doesn’t work by averages…. it only works under specific conditions. If you have a low wind day with a hot sun, and a low wind night, the town will be warmer. If a wind comes up at night with a low temperature, you go almost as low as the suburbs fairly quickly.
You can see variations in temperature just driving a few miles. Some places are shaded by hills and don’t warm easily. Sometimes in winter, the bottoms of hills will freeze first. THe suburbs of Baltimore are anything but that, at this point. You have lots and lots of asphalt, huge traffic problems, and businesses popping up along major roads. So, the conclusion that the heat island of the suburbs is an extension of the city’s heat island is false. The suburbs would generate their own heat island even if the city were wooded.
The map that they have is a growing season map. The season length depends upon when you have a freeze. The longest seasons are near the water (no surprise) and the shortest seasons are on the tops of hills (get coldest first). This business about suburbs receiving the city heat island effect is nonsense and could easily be shown to be false by overlaying a season length anomaly map with a map of new construction along the I-95 corridor leading out of Baltimore. You will get a perfect match. I haven’t done this calculation, but, I have watched the relentless development of former farm & marsh land over the last 15 years.
Do me a favor and call BS on this with these authors. Ask them to overlay development with growing season anomaly.

Wow! The UHI contamination is worse than previously thought. Hmmmm … I see an opportunity for some ambitious grad student to do a ground-breaking study of UHI contamination that oppotunistic climate “scientists” can jump on and declare, “We were duped by the data!”, and then REALLY focus on accidification….

My family has been in the area for more than a century and have had the misfortune of watching hordes of people move to Washington, D.C. to feed at the government trough. Sixty years ago, D.C. was a backwater where no self-respecting person would want to live. It was surrounded by some of the most gorgeous countryside imaginable. With the nearly unchecked growth of government, that countryside has been wrecked by hideous development.
Natives joke that the original motto of the Chesapeake Bay Foundation (c. 1960), “Save The Bay” should be replaced by a more accurate motto, “PAVE THE BAY.”

Apologies for the off-topic question, or if this serious issue has been dealt with somewhere else (impossible to read it anyway!), but what’s up with the the pale grey, all-caps and sans-serif microscript in the comments section? It’s as if someone combined all typographical boo-boos which make texts unreadable. Screen reading speeds are already lower by as much as 30% compared to traditional black type on white paper and the added tiny point size, harder to read sans-serifs and the low contrast between type and screen background make me suspect a psy-ops from the Warmies. Would have DeSmog’s mark of “sophistication,” methinks.
On the other hand, I think this alarming typographical blight appeared first on Willis’ article. That’s enough evidence to collectively blame Willis, I rather than to flog the Warmist dead horse. Blaming Willis is a hallowed custom here and results in colourful zingers from him which make for much better reading then the anemic whining and sniffling from the Warmies.

Indeed, you have “pierced the corporate veil” and placed the blame exactly where it belongs, on my head. Or rather my eyes. And since your analysis has been so accurate, I will spare you my usual imprecations and speculations on your ancestry and dining habits …
I thought I too was having trouble with script shrinking, and that my arms were getting shorter when I held something out at arms length to read it … but it was just my eyes.
In any case, Peter, on my Mac at least the key combination “command +” makes the page larger, and “command -” makes it smaller. Or it’s under the “View” menu in Safari.
WordPress is kinda funny in that they have various “themes”, combinations of fonts and styles and such. Not all combinations are available, so often folks end up having to compromise.
All the best,
w.

Larry Gieger – good point. Where I live in the far northeastern corner of California, one only has to drive 25 miles to the east over the Warner Mountains into Surprise Valley to enjoy an extra month of growing season, same elevation, same humidity, but slightly warmer.

Bill says:
March 9, 2012 at 7:47 am
I wonder if trees put more water out than they take in? They split water to make O2 as part of photosynthesis.

Briefly, photosynthesis comprises 2 processes, one light dependent and one light independent.
In order to synthesize 1 molecule of glucose (C6H12O6), plants split 12 water molecules (12 H2O) releasing 6 O2 molecules and keeping the 24 hydrogens as 12 molecules of NADPH2 and energy as ATP. That happens during the light dependent phase.
In the light independent phase, (Calvin cycle) plants capture 6 molecules of CO2, and they use the 12 NADPH2 molecules and energy formed in the previous phase to synthesize 1 molecule of glucose (C6H12O6) and they release 6 H2O molecules.
If you look at the photosynthetic process as a whole thing, plants consume 6 H2O molecules per molecule of glucose synthesized.
Then, if plants use glucose to synthesize starch or cellulose, they release 1 water molecule per molecule of glucose added to the cellulose or starch chain. But still, as a whole, plants consume water.
But they may increase the rate ground water evaporates, I don´t know about that.
The capture of CO2 is a passive process, Depends on the concentration of CO2 in the atmosphere, The higher the concentration, the faster the synthesis of glucose. That´s why it is good to increase the atmospheric CO2 levels.

Peter Hartley says:
March 9, 2012 at 7:42 am
“Although the article does not directly say it, it is also likely that increased CO2 concentration in urban areas has played a role in extending the growing season. It cannot be said because that gets too close to admitting that additional CO2 has some beneficial effects.”
The extended growing season was observable in Bournemouth (pop. approx 150,000) back in the 70s and this is before significant increases in road vehicles. It was very evident in London with as much as a 3 week difference of start and end to the season. Now, this is when I first noticed not when the effect first started. I’m not saying that CO2 is not a food, just that CO2 won’t determine the start or end. Consider the walled kitchen garden, which have been around for centuries and provide shelter for whatever is cultivated there:http://en.wikipedia.org/wiki/Walled_garden
“The shelter of walling can raise the ambient temperature within the garden by several degrees, creating a microclimate that permits plants to be grown that would not survive in the unmodified local climate. Most walls were constructed from stone, but lining walls with brick, which absorbs and retains solar heat, raised the temperature against that wall, allowing peaches, nectarines and grapes to be grown as espaliers against south-facing walls as far north as southeast Great Britain and southern Ireland.”
This is horticultural knowledge so I imagine climate researchers wouldn’t consider asking gardeners about climate.

Well, thanks for your reply, Willis, and in the spirit of full disclosure, the truth behind my ancestry and dining habits is far more shocking and scandalous than anything you can come up with. It’s worse then you thought, to borrow a well-worn and charming phrase. But rumour has it you can best me in that department too.
There two mysteries which still remain, now that you reminded me of the existence and whereabouts of my text size command (seriously). The first is, why the new and old formats keep switching unpredictably when refreshing a page, the other one is why, with the pack of noisy malcontents here, no one else has either noticed or complained. This began happening right after my post, and due to the effects of my post-Purim hangover and congenital paranoia, I concluded that you were messing with me because…well, because you can. Now I understand that you’re trying to fix the issue and you have my sympathies; I enjoed the early days of html-based web design on DreamWeaver, but with all the CSS, coding, glitchy and half-arsed “themes” and other annoying bells and whistles, I leave such to my colleague and concentrate instead on words and art. It’s better that way for everyone.
Since I (possibly) have your attention here, I take this opportunity to thank you for the hours of pleasure and education your contributions here have provided me with. Being a liberal arts type and an artsie, most articles of yours fly entirely over my head…at trans-Atlantic flight altitudes… and yet I read every single one because I get a kick out of your logic and style and because I always come away with just a little bit more knowledge than I came in with. Keep up the good fight, Gladiator, I salute you!
Have a good weekend!

Bill says:
March 9, 2012 at 7:47 am
“I wonder if trees put more water out than they take in?”
Although Urederra (March 9, 2012 at 11:17 am) is correct on the phytochemistry, the issue with trees and water is mostly transpiration – movement of water from the soil via the roots and out of the leaves as H2O and not via any chemical reactions.
Trees have to open pores in their leaves to allow entry of CO2 and this allows a lot of water out (the leaf outer layers are often impervious to evaporation). The movement of water out of the leaf essentially ‘pulls’ water up vascular tissues from the roots and if there is too little water in soil, the rate of flow stops, the plant ‘wilts’ and the pores in the leaves close (switching of CO2 fixation in the process). This accounts for most of the water movement as the actual chemical needs of photosynthesis are not all that great.
Interestingly enough, the number of pores (called stomata) seems to be linked to atmospheric CO2 levels – lower CO2 means more stomata. I think this is one of the proxies people have tried to use for historic CO2 levels, but there are probably a number of confounding factors as well. It also explains why plants are more productive under high CO2 levels and suffer less water stress when CO2 is higher (better able to withstand low soil moisture levels).
Sorry to be nerdy, but I thought this would be a useful point.

This study needs to be repeated in other areas. Urban effects may be more significant and in different ways than we expected; but it’s hard to say how and to what extent with just one region reporting.
I would love to see Mosher’s analysis, and what would happen if we increase the “ring” zone of “urban” definitions (that is, that cities create an extended microclimate zone of temperature/precipitation/other weather effects different from actual climate).

Peter Hartley says:
March 9, 2012 at 7:42 am
Although the article does not directly say it, it is also likely that increased CO2 concentration in urban areas has played a role in extending the growing season. It cannot be said because that gets too close to admitting that additional CO2 has some beneficial effects.
================================================================
How would CO2 concentrate in an urban area to the degree that there would be any impact?

(Alleged) problems can also be turned to advantage.
There’s an occasional, and entertaining, series on BBC radio in which comedian Mark Thomas invites proposals from the audience for inclusion in ‘the manifesto’. The most nearly plausible are discussed between Thomas (an intelligent revolutionary), the proposer and, usually, the audience, with voting (by applause) at the end of the show.
A few weeks ago, one proposal was accepted immediately and just waved through at the end. It is magnificently simple. Suggestion: All local councils should be required to plant fruit trees in public places, rather than the ones they plant now.
And if people nick the fruit? – well, that’s the whole point. It certainly got my vote on the spot. We know ever more about the extent of our ‘heat islands’, let’s get on with using that energy sensibly.We know where it is and what it can do.
It’s also rather pleasing to note that they mentioned during the discussion that some cities in Britain are already planting fruit trees. 🙂

Noticed EXACTLY this today. Red maples blooming in a Mall parking lot. Red maples are common my rural area — only 150′ above in elevation, but way behind the city-slickers. Guessing 2 more weeks here before they (& Silver maples) bloom.

I recall seeing some years ago driving down a tree-lined street in winter with all the trees having lost their leaves, except for little clusters of green leaves at the same height and at regular intervals down the street. Since it was daytime, it took me a while to twig that each cluster of leaves was around a street light. I remember thinking at the time that I was surprised light could make such a difference to winter dormancy patterns, but probably it was more the heat than the light, street lights in those days were all incandescent.
I’m surprised these authors remark on a carryover of heat from summer to winter. Surely the massive amount of waste heat released by human activity in cities would be enough? As noted in comments above, a cold wind can drop even warmer microclimates within a city to low temperatures within hours.

****beng says:
Your comment is awaiting moderation.
March 9, 2012 at 12:01 pm
****
Oh yeah, the city where the above Red maples were located has an official population of ~21,000. Downtown is usually 2-6F warmer than my rural location, from the car thermometer.

Anyone with a newer car these days can tell you UHI absolutely does exist. I regularly drive from a semi rural area to the core of a downtown area and back – appx 30 miles each way. There is regularly as much as a 2 to 4 degree F temp variation within this distance. This is especially pronounced as temps drop below zero in winter, and on the hottest days of summer.
Much of the alleged global warming, as I recall, has been in higher average night time lows. This is exactly what I see as I watch the temp drop on those cold winter nights.
Would be interesting to see how the WUnderground network of personal weather stations could be used? Maybe a “verification” program where they sent a picture of the siting and address which could be reviewed and assigned a quality rating. I can look at the WunderMap for my area and see it is 34-35F downtown, generally appx 32F in suburbs, and about 29-31 in immed surrounding rural areas.

This image shows not only the urban heat effect, but also how trees and water are ‘cold’: http://wattsupwiththat.files.wordpress.com/2011/04/3_left_london_uhi1.jpg
The blue/dark features clearly define known woods or lakes, and of course the Thames. Added to that these features are themselves dark, or supposedly heat absorbing.
The reach of UHI out into rural areas may be driven by that urban areas water requirements and how much water is extracted or what drainage is introduced. What was a surprise for me in this photo was the relative warmth of grass areas in Greenwich, Hyde Park and Richmond Park, while also containing the coolest areas, which are the trees and water. I can only explain this in that these parks are relatively high and the grass is short, so the ground will dry out quickly. Also, this photo was taken in September 2003. That summer was hot in the UK and the ground dry, However, this is image was taken in September, so these are not hot summer temperatures.

The other thing that occurs to me is temps are affected significantly by things like snow cover. I would think a winter without snow will show higher temps – if you do not have the cooling effect of snow. Plus UHI could cause less snow remaining on ground in some areas. Seems to make winter temps linked to precipitation at least in part.

Over the last 100 years urban areas have grown at a rate that is staggering and has no historical parallel. 100 hundred years ago there were a handful of cities over 1 million. Now there are over 350 and rising. Considering some cities have tens of millions of people, the urban sprawl, plus the enormous draw on ground water from surrounding areas (Lake Owens, Colorado River) probably explains not some, but all of the observed warming over the last 100 years.
How is this not significant?

Where was Baltimore’s UHI in 2011? I live in Churchville, Maryland (within 20 miles of Baltimore as the crow flies) and last year we had a very cold and wet spring – so cold and wet our strawberries, tomatoes and other fruits and veggies did very badly. Maybe some more research is needed.

“The study found that urban heat islands affected the growing season in areas within 20 miles of the city.”
How do urban heat islands affect the growing season but not the average temperature? Maybe someone should send a copy of this paper to Richard Muller and the BEST team who claim that urban heat has had no affect on global temperature records.

We are missing out on a prime opportunity to point out a major blunder when statements like “a greenhouse gas that traps heat” are used, when referring to CO2. We need an absolute minimum of at least 110 to 120 ppm or so of the gas just so we don’t run out of oxygen and food, for at that low level plant life stops growing and starts to die off, and at that minimum level, essential just to barely sustain life on this planet, most of the heat trapping has taken place when compared to the current level of CO2. So when one says we are adding heat trapping gas, this is not true when the effect is the additional gas is considered instead. We have got to shout this out, make it clear that most of the heat trapping is with the minimum levels that we absolutely must have for sustained life on this planet, whenever some one uses the words “heat trapping” to try to win their alarmist arguments.

This is actually a very lovely experimental technique. It is unfortunate that the usable data is rather sparse (all the gray areas) but that simply means more sites need to be examined.
Repeating this in as many urban areas as possible may make it possible to produce a rather nice “averaged” effect of various size cities. Possibly, some distortions will need to be corrected for each city. I suspect that Baltimore and DC are unusually “circularly symmetric”, and other cities will be much less so, and some diddling will be needed to gain as much information as possible. What I would hope to be able to generate from such a major study would be “contour lines” of growing season changes varying with population size. Hmm… it is also likely that some adjustment function will be needed for different base temperature as well as size. I wonder what other adjustments will be required.
Anyway, after generating such a “contour line” of growing season difference versus size, it ought to be simple to translate that to the effective average temperature difference based on distance from that size city using standard and well known gardener’s growing season vs. temperature charts.
Voila – a way to systematically correct historical temperatures near growing cities based on experimental method and data instead of mere hand-waving and “homogenization” … well, at least it would have some real science arguments instead of mere statistical processing to back it up.

Shame on you skeptics! Correlation is not Causation.
Perhaps there are Natural Heat Islands that attract people to build Urban Areas.
What’s so hard to believe? Harbors attract people so that cities get built around them. 😉

I’ll suggest 3 possible causes other than increasing temperatures.
Increased precipitation. Its well known that precipitation increases in and around cities. One study in China found that after they had built a city from scratch, precipitation rose 300% in a nearby forest.
Increased CO2 availability. There is more CO2 available in cities. There have been several recent studies that CO2 is the limiting factor in plant growth.
Increased solar insolation. There have been large reductions in urban aerosols in the developed world. These aerosols both scatter sunlight and seed clouds. Reduce them and you get more solar insolation.
I think the third is probably the main cause of the increased growing season.

Jay Davis says:
March 9, 2012 at 2:31 pm
“Where was Baltimore’s UHI in 2011?”
You have a point and JDN above offers his explanation that the UHI is dependent on prevailing conditions. However. over a year the average temp will be higher in urbanised and agricultural land. On hot days the heat coming off buildings, roads and cultivated land can be spectacular. This is not heat that is trapped or going round a second time by some feedback, it’s simply exposed surfaces that are good at absorbing sun heat then warming up the air above it by contact (/lecture).

It doesn’t even take a mile. I live in an apartment complex, with tomato and flower gardens between two of the buildings, and a garden in the large back yard behind the parking lot, about 100 feet away from the buildings. Winter comes much later to the between-buildings garden, and Spring comes weeks sooner.

Following is a peer-reviewed article on how the urban heat island effect impacts opossums–extending their range northwards:
Kanda, L. Leann, Todd K. Fuller and Paul R. Sievert, 2006. Landscape Associations of Road-killed Virginia Opossums (Didelphis virginiana) in Central Massachusetts, Am. Midl. Nat 156:128-134.
Humans have a beneficial effect on some wild creatures, but tend to reduce biodiversity quite sharply. We are learning now how to improve this. A nice example is the river that flows through downtown Denver–the Platte. Ten or twelve years ago, it was filled with old bedsprings and other trash.It was cleaned up and native plants were planted. Now it is home to several species of wild ducks, red-winged blackbirds, bluejays, and other birds, foxes and other creatures.
The coming greenie shift from cAGW hysteria to a focus on biodiversity could be a very good thing. We can indeed increase biodiversity and at a reasonable cost. But unless the greenies learn to reason better than they have about the climate, they cannot do anything right.

Lady Life Grows says:
March 10, 2012 at 7:16 pm
It doesn’t even take a mile. I live in an apartment complex, with tomato and flower gardens between two of the buildings, and a garden in the large back yard behind the parking lot, about 100 feet away from the buildings. Winter comes much later to the between-buildings garden, and Spring comes weeks sooner.
Many apologies for repeating this but I mentioned the walled kitchen gardens earlier, which have been around for centuries and provide shelter for whatever is cultivated there:http://en.wikipedia.org/wiki/Walled_garden
“The shelter of walling can raise the ambient temperature within the garden by several degrees, creating a microclimate that permits plants to be grown that would not survive in the unmodified local climate. Most walls were constructed from stone, but lining walls with brick, which absorbs and retains solar heat, raised the temperature against that wall, allowing peaches, nectarines and grapes to be grown as espaliers against south-facing walls as far north as southeast Great Britain and southern Ireland.”

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